By Donna Fleury
Researchers screen and evaluate potential bacterial strains for foliar-applied biopesticide.
By Donna Fleury
Sclerotinia stem rot, caused by the pathogen Sclerotinia sclerotiorum, is one of the most destructive diseases of canola. The severity of sclerotinia stem rot is extremely variable from year to year, region to region and even from field to field depending on weather conditions, and can be challenging to manage. If warranted, timely chemical foliar fungicides can be used, but there are currently no resistant cultivars available. Researchers are developing and evaluating biopesticides as another control option for this disease.
“The S. sclerotiorum pathogen can cause disease on a wide range of over 400 plants and is a major economic problem worldwide,” says Susan Boyetchko, research scientist with Agriculture and Agri-Food Canada in Saskatoon. “This is a very successful pathogen and it is tricky to find the best way to control sclerotinia disease.
“In collaboration with other researchers, we have a five-year research project underway to screen and evaluate the biopesticide potential of selected bacterial strains and determine their ability to control disease development and growth of S. sclerotiorum in canola.”
Boyetchko’s research program focuses on biopesticides, which use living organisms and/or their natural products to biologically control weeds, plant diseases, and insect pests. They are environmentally friendly and contribute to the endurance and environmental performance of the ecosystem. In her lab, she has assembled a diverse culture collection of almost 3,000 bacterial strains, all selected from the Canadian Prairies. Through her program, she has successfully released a bioherbicide for grassy weeds and has recently developed a biopesticide to control late blight in potatoes, which is currently being commercialized by an industry partner. Sclerotinia stem rot is a new priority in her program.
“We have already identified a few bacterial isolates that are inhibiting sclerotinia in different ways,” Boyetchko explains. “This project is taking a multifaceted approach to test these promising bacterial strains to help explain how they can be responsible for biological control of sclerotinia in terms of microbial and plant genomic studies, as well as plant defence systems.
“We want to characterize the different bacterial strains of the biopesticide and look genetically at what causes the improvement of canola to sclerotinia. We also want to understand how a plant uses the biopesticide as a defence against the pathogen.”
Researchers have found that not all biopesticides have the same modes of actions. One mode of action induces resistance by triggering a key enzyme or compound, such as protein or phytoalexins, which are antimicrobial and antioxidative substances. These are broad spectrum inhibitors and are chemically diverse, with different types characteristic of particular plant species. Similar to developing antibodies, plants are able to synthesize these compounds and rapidly accumulate them at areas of pathogen infection.
“Therefore, we want to understand if these selected bacterial strains behave differently in different parts of the S. sclerotiorum pathogen lifecycle and what their mode of action is,” she adds. “In particular, we are focusing our approach on the ascospores, or airborne sexual spores, that are the primary inoculum causing disease in the growing canola crop. Because they are airborne, ascospores can move in the wind for several kilometres and, under the right conditions, infect new and neighboring fields. Other previous research approaches have looked at controlling the overwintering sclerotia using soilborne strategies.”
In the first two years of the project, a few bacterial strains have been identified and evaluated. Boyetchko notes that much of the testing is being conducted in the lab and in greenhouses. In the trials, conditions are set for the sclerotinia pathogen to develop optimum inoculum levels, followed by biopesticide application under both good and harsh or stressful conditions to see how the biopesticide organism behaves. All of the bacterial strains in Boyetchko’s collection have been isolated from very cool Prairie conditions and are able to grow at 10 to 15 C or higher, and hopefully under any environmental conditions that are more optimal for biopesticides.
“We have selected and tested five bacterial strains to date as a foliar biopesticide application, and all were found to inhibit ascospore germination, mycelial growth and sclerotial formation of S. sclerotiorum,” Boyetchko explains. “In one trial, all plants sprayed with the bacterial strain PENSV20 in the presence of the pathogen had no symptoms of the disease, and plant defence genes were triggered when sprayed 24 hours before and 24 hours after the pathogen.
“We will continue the biopesticide evaluation research for the next three years of the project, with plans to move testing out of the greenhouse and into field trials in the final year. The next step for this study will be the development of a foliar-applied bacterial biopesticide to be used in integrated pest management.”
Boyetchko’s research team is following a biopesticides innovation roadmap they developed a few years ago. This is a step-by-step approach to evaluating and understanding both the biopesticide organisms and the pathogen.
“We don’t want to rush the process and we want to make sure we fully understand the potential of the biopesticide – how to best produce and formulate it, what the application requirements are and overall what is the best approach,” she says. “We want to make sure we understand the safety of all aspects of the biopesticide – to the crop, to the environment and to the growers – before we put it into growers’ hands. Similar to any new pesticide product, chemical or biological, this includes the product registration process under the Pest Management Regulatory Agency.
“Should we be successful in registering a new product and getting it into the integrated management toolbox, then we could look at potential label expansions and testing on other crops. We are ultimately looking for the best of the best, and hopefully in the future we’ll be able to commercialize a new biopesticide for sclerotinia in canola.”